Lens.es6.js

import { compose, curry, map } from 'ramda'
import Const, { getConst } from './functors/Const'
import Identity, { runIdentity } from './functors/Identity'
import K from './functors/K'


/* ----------------------------------------- *
        Lenses définition
* ----------------------------------------- */

// objectLens :: String -> Functor f -> Object -> f Object
const objectLens = curry((key, f, obj) => map(value => ({
  ...obj,
  [key]: value
}), f(obj[key])))


// num :: Number -> Functor f -> [x] -> f [x]
const num = curry((index, f, arr) => map(value => [
  ...arr.split(0, index),
  value,
  ...arr.split(index + 1)
], f(arr[index])))


/* ----------------------------------------- *
        The 3 methods
* ----------------------------------------- */
// s = data structure
// a = value of a specific key or index
// Lens s a = Lens of a defined data structure (Object, Array...) and a defined key or index

// view :: Lens s a -> s -> a
const view = curry((lens, x) => compose(getConst, lens(Const.of))(x))

// over :: Lens s a -> (a -> a) -> s -> s
const over = curry((lens, f, x) => compose(runIdentity, lens(compose( Identity.of, f )))(x))

// set :: Lens s a -> a -> s -> s
const set = curry((lens, v, x) => over(lens, K(v), x))



/* ----------------------------------------- *
        Let's play
* ----------------------------------------- */

const user = { name: 'Gabriel', age: 22, friends: [{ name: 'Stachmou' }, { name: 'HugoGuiillouu' }] }

const L = {}
L.name = objectLens('name')
L.friends = objectLens('friends')
// lenses compose left to right
L.secondFriendName = compose(L.friends, num(1), L.name)


console.log(view(L.name, user))
// => Gabriel

console.log(view(L.secondFriendName, user))
// => HugoGuiillouu

console.log(over(L.name, (name) => `${name} Vergnaud`, user))
// => { name: 'Gabriel Vergnaud', age: 22, friends: [...] }

console.log(set(L.name, 'Gabz', user))
// => { name: 'Gabz', age: 22, friends: [...] }